Project Summary Mitochondrial function declines during aging and is accelerated in age-associated diseases such as Parkinson's and Alzheimer's. Thus, understanding how cells and organisms generate the appropriate mitochondrial mass during development and maintain that mitochondrial network during aging is an essential step to limiting pathologies associated with loss of mitochondrial function. Here, we aim to understand the underlying mechanisms by which the transcription factor ATFS-1 and the mitochondrial unfolded protein response (UPRmt) coordinate mitochondrial network biogenesis during normal development and the recovery of the mitochondrial network via independently regulated functions of ATFS-1 in mitochondrial compartments and the nucleus. Numerous components have been identified that promote replication of the mitochondrial genome and transcription of nuclear-encoded genes that result in mitochondrial biogenesis. However, it remains unclear how the appropriate amount of mitochondrial mass is achieved during development or cell specification, and how the mitochondrial network is maintained over a cell or organisms lifetime. It also remains unknown if mitochondrial genome (mtDNA) replication is coordinated with cell growth, physiology, or functional heterogeneity within the compartments that comprise the mitochondrial network. Current dogma suggests that mtDNA replication is entirely dependent on expression of several nuclear-encoded factors such as the mtDNA polymerase. Despite functional heterogeneity within the mitochondrial network, compartment-specific autonomy over mtDNA regulation is rarely considered. In this proposal, we aim to understand how ATFS-1 establishes the appropriate amount mitochondrial mass during development, and maintains the mitochondrial network over an animals lifetime focusing on the separate activities of nuclear ATFS-1, and the fraction of ATFS-1 that accumulates in dysfunctional mitochondria and binds mtDNA. Lastly, we aim to understand the impact of an ATFS-1-mediated diapause that occurs if the mitochondrial network is severely damaged during development. At the end of the funding period, we hope to understand the endogenous strategies in place to establish and maintain the mitochondrial network, to potentially develop strategies to promote or maintain a robust mitochondrial network.